add disconnected auth flow

[mwfarb]

We may have use cases where we want to keep ARENA management completely in a closed system, like a digital twin system for factories. In that case we'd need to support a side loaded user account create/recovery locally. So we could keep the allauth definitions, but need to then support an option where users are performing standard logins. From the UI perspective, we'd add "Login with Email" and remove "Login with Google", but that could be deployment configurable.

[mwfarb]

Somewhat tangential is the option to "Login as ARENA", where we could support our own OAuth: https://github.com/openauthjs/openauth.